Polystyrene produced by conventional radical polymerization is amorphous and has an atactic configuration. It is, however, possible to produce other forms of polystyrene with more stereoregular configurations such as isotactic and syndiotactic polystyrene. As the degree of syndiotacticity increases, the melting point rises and the material has excellent heat and chemical resistance.
Syndiotactic polystyrene is polystyrene wherein the phenyl groups pendent from the chain alternate with respect to which side of the chain the phenyl groups are pendent, i.e. every other phenyl group is on the opposite side of the chain. Isotactic polystyrene has all of the phenyl groups on the same side of the chain while atactic polystyrene, which is the standard or normal, has the phenyl groups randomly oriented with no discernable pattern.
Syndiotactic polystyrenes may be prepared by polymerization using any one of numerous catalysts based on a variety of titanium, zirconium, and hafnium compounds. One such catalyst system has been described by Pellecchia et al in Macromol Chem. Rapid Commun. 13, 265-268 (1992) and is based on Cp*Ti(CH.sub.3).sub.3 where Cp*=.eta.-pentamethylcyclo pentadienyl, activated with tris(pentafluorophenyl)boron [B(C.sub.6 F.sub.5).sub.3 ], in a toluene solvent, which was used at a temperature of 50.degree. C. to produce less than 50% yield of syndiotactic polystyrene. It is possible that Pellecchia et al failed to obtain the syndiotacticity and yields reported in the present application because they (a) did not maintain the solvent catalyst and co-catalyst rigorously anhydrous and anaerobic, and/or (b) did not completely remove all traces of styrene inhibitor from the styrene starting monomer. By "rigorous anhydrous and anaerobic conditions" is meant the solvent is dried by refluxing over sodium-benzophenone followed by distillation under nitrogen. The catalyst is purified by recrystallization from hexanes and is stored only briefly under nitrogen at -25.degree. C. The co-catalyst is purified by sublimation and stored under nitrogen at -25.degree. C. Attention is also directed to U.S. Pat. Nos. 4,978,730; 5,045,517; 5,132,380 and 5,169,893 which describes alternate catalyst systems for the production of syndiotactic polystyrene and other olefins.
It has now been determined that very much higher yields of an amorphous syndiotactic polystyrene at high rates of production can be achieved by careful temperature control below about 70.degree. C. The rate of reaction and final yield (i.e. ratio of finished polymer to starting monomer) can be controlled so that the process is amenable to reaction injection moulding (RIM) of hard, rigid, finished product which maintains the shape of the reaction vessel. It has also been determined that the ratio of catalyst to co-catalyst and the type of catalyst solvent employed markedly influence the degree of syndiotacticity achieved.
It will be appreciated by those skilled in the an that yields in the order of 50% are insufficient to produce a hard rigid product as too much liquid monomer is retained in the final product.